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(a) Flux paths in squat core

Figure 4.4 Cross flux at corners forms greater portion of total fluxpath in short squat core than in tall slim core

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these again at 1.5 T. In the case of POWERCORE@ strip this has risen to1.3 VA/kg while for conventional silicon steel it is typically only 0.94 VA/kg.

While the sizes of strip available as POWERCORE@ are still unsuitable for

the manufacture of large-power transformer cores, in the USA in particular,

many hundreds of thousands of distribution transformer cores with an average

rating o f around 50 kVA have been built using amorphous material. In Europe

use of the material has been a far more limited scale, the main impetus being

in Holland, Sweden, Switzerland, Germany and Hungary. One possible reason

for the slower progress in Europe is that the thin strip material does not lenditself to the European preferred form of core construction, whereas the wound

cores, which are the norm for distribution transformers in the USA, are far

more suitable for this material. In the UK its use has been almost exclusively

by one manufacturer who has built several hundred small distribution trans-

fonners. All were manufactured from plain un laminated ribbon material. This

manufacturer has also built a small number of experimental units using the

POWERCORE@ material, see Figure 3.8, but report that the difficulties of

cutting and building this into a conventional core can tend to outweigh any

benefits gained.

Another of the practical problems associated with amorphous steel is its poor

stacking factor which results from a combination of the very large number

of layers of ribbon needed to build up the total required iron section and


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2 Design fundamentals

There are two basic types of transformers categorised by their winding/core

configuration: (a) shell type and (b) core type. The difference is best under-

stood by reference to Figure 2.1.


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three-limb arrangement. With this configuration, having top and bottom yokesequal in cross-section to the wound limbs, no separate flux-return path is neces-

sary, since for a balanced three-phase system of flux.es, these will summate

to zero a t all times. In the case of a v ery large transformer which may be

sub ject to height limitations, usually due to transport restrictions, it may be


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The LV winding leads are taken out at the top and bottom of the leg, which

means that they must of necessity pass close to the core framework. Since

they are at relatively low voltage, it is probable that the necessary clearance

can be obtained by .bending these away from the core as close to the winding

as possible and by suitably shaping the core frame (Figure 4.26(c)).

The HV winding leads also emerge from the top and bottom of the leg but

these are taken on the opposite side of the coils from the LV leads. Being at a

greater distance from the core frame than those of the LV winding, as well as

having the relatively modest test voltage of 70 kV, these require a little more

insulation than those of the LV winding.

It is usually convenient to group the tapping sections in the centre of the HV

windings. This means that when all the taps are not in circuit, any effective

'gap' in the winding is at the centre, so that the winding remains electromag-

netically balanced. More will be said about this aspect below. The tapping

leads are thus taken from the face of the HV winding, usually on the same

side of the transformer as the LV leads.

Figure 4.27 shows the arrangement of a transformer in which the LV

winding is fully insulated and the HV winding has non-unifonn (graded)

insulation. This could be a bulk supply point transformer, say, 132/33 kV,

star/delta connected, possibly 60 MVA, belonging to a Regional Electricity

Company (REe). Some RECs take some of their bulk supplies at 11 kV,

in which case the transformer could be 132/11 kV, star/star connected, and

might well have a tertiary winding. This too could be 11 kV although it is

possible that it might be 415 V in order to fulfi Ithe dual purpose of acting as


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The LV winding leads are taken out at the top and bottom of the leg, which

means that they must of necessity pass close to the core framework. Since

they are at relatively low voltage, it is probable that the necessary clearance

can be obtained by .bending these away from the core as close to the winding

as possible and by suitably shaping the core frame (Figure 4.26(c)).

The HV winding leads also emerge from the top and bottom of the leg but

these are taken on the opposite side of the coils from the LV leads. Being at a

greater distance from the core frame than those of the LV winding, as well as

having the relatively modest test voltage of 70 kV, these require a little more

insulation than those of the LV winding.

It is usually convenient to group the tapping sections in the centre of the HV

windings. This means that when all the taps are not in circuit, any effective

'gap' in the winding is at the centre, so that the winding remains electromag-

netically balanced. More will be said about this aspect below. The tapping

leads are thus taken from the face of the HV winding, usually on the same

side of the transformer as the LV leads.

Figure 4.27 shows the arrangement of a transformer in which the LV

winding is fully insulated and the HV winding has non-unifonn (graded)

insulation. This could be a bulk supply point transformer, say, 132/33 kV,

star/delta connected, possibly 60 MVA, belonging to a Regional Electricity

Company (REC). Some RECs take some of their bulk supplies at IIkV,

in which case the transformer could be 132/11 kV, star/star connected, and

might well have a tertiary winding. This too could be 11 kV although it is

possible that it might be 415 V in order to fulfi Ithe dual purpose of acting as


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built up by winding outwards exactly as the first. When this second complete

disc has been formed, the tension is taken off the winding conductor, the taper

former removed and the turns laid loosely over the surface of the mandrel.

These turns are then reassetnbled in the reverse order so that the 'start' is the

crossover from the adjacent disc and the 'finish' is in the centre at the mandrel

surface. The next disc can then be built upwards in the normal way. A sectionof continuous disc winding is shown in Figure 4.18.

Figure 4.18 Arrangement of continuous disc winding


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stacKIng ractor wnIcn results rrom a COITIOlnanOnor tne very large numoer

of layers of ribbon needed to build up the total required iron section and

Figure 3.8 Core and windings of ?OO kVA, 20/0.4 kV transformer

using amorphous steel. Unfortunately very little of the core is

visible, but it should be just apparent that this is of the wound

construction. It will also be apparent that fairly elaborate clamping

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